http://experiments.chary.us/gamercard/ This thing lets you make dynamic signatures and gamercards for use on forums and websites and such. It is still in beta. Some gamertags might not work as expected. If anything bad/unexpected happens or you have an idea for a feature/theme, let me know here. I am looking for comments and suggestions.
And as for some ideas for designs, get some new shapes in there. As for backgrounds, take a look around for the most popular games that are out now or coming out soon, and put them as the background. Take a look around at all the other sites that are producing gamercards, and see what they are offering. Maybe even trawl 360 forums and see which gamercards are been used the most.
@1st gtaROCKER: Click the link in the first post. Enter your gamertag. Select a theme. Press enter. Copy the applicable code to where you want to use it. @GTArv: Already do. Gamertag info and avatars are cached for one hour and game images are cached forever. @Butters 2011: Alright.